This invention relates to a device for generating a light spot control signal and more particularly to a device which generates a control signal for jumping a light spot from one track to another.
An optical reader or a so-called light disk apparatus has hitherto been proposed wherein a light spot such as a laser beam serving as a read beam is irradiated on a disk-shaped recording medium having a spiral track or concentric tracks on which information is recorded in the form of topographical irregularities responsible for change of reflection factor, and the information is read by detecting changes in light quantity of reflection or transmission light. In this type of light disk apparatus, there is required a control operation for jumping the light spot. For example, with a recording medium having a continuous spiral a portion of which is a spiral track recorded with a television signal, a still picture can be reproduced by jumping back a light spot radially over a length of one track each time the recording medium makes one revolution, namely, scanning an identical track with the light beam during a required time interval. In another instance, a slow motion operation at a speed which is half the rated speed can be accomplished by jumping back the light spot radially over the length of one track each time the recording medium makes two revolutions. With a recording medium having a great number of concentric tracks, a light spot is caused to jump radially from one track to another.
One example of such a jump control method is disclosed in Japanese Patent application Laid-Open No. 52-26802. This jump control method will be described with reference to FIG. 1 which illustrates at sections (a) through (c) a track error signal waveform, a control signal waveform and a cross-sectional form of tracks. In this jump control method, based on the fact that a track error signal E.sub.1 representative of a displacement between a light spot and a track becomes zero, as shown at A' and C', at centers A and C of tracks and, as shown at B', in the middle of adjacent tracks, the polarity of a control signal E.sub.2 is changed by detecting the point B' at which the track error signal becomes zero, a time interval T required for excursion of the track error signal from point A' to point B' is measured, and a duration time T' of the inverted-polarity pulse is made equal to T. The control signal E.sub.2 thus obtained is applied to a tracking device such as a deflector.
Incidentally, when the deflector is driven by application of a pulse of a pulse height or pulse level E.sub.A to move the light spot present at point A to point B, the light spot begins to move toward point B. However, such a movement of the light spot is adversely affected by local friction in the deflector or irregular sensitivity of the deflector to a drive voltage (or drive current) for making a deflection angle, and it happens that the light spot fails to move or it returns to the point A in the course of its movement toward the point B. Even with a deflector not raising the above problem, a similar phenomenon occurs when the disk per se is moved in opposition to the light spot under the influence of external vibrations. In such an event, the aforementioned method which merely detects the zero point of the track error signal is unaware of the return of the light spot.